1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method of operating a relay station in a wireless communication system.
2. Related Art
Institute of electrical and electronics engineers (IEEE) 802.16 standard provides a technique and protocol for supporting broadband wireless access. The standardization had been conducted since 1999 until the IEEE 802.16-2001 was approved in 2001. The IEEE 802.16-2001 is based on a physical layer of a single carrier (SC) called ‘WirelessMAN-SC’. The IEEE 802.16a standard was approved in 2003. In the IEEE 802.16a standard, ‘WirelessMAN-OFDM’ and ‘WirelessMAN-OFDMA’ are further added to the physical layer in addition to the ‘WirelessMAN-SC’. After completion of the IEEE 802.16a standard, the revised IEEE 802.16-2004 standard was approved in 2004. To correct bugs and errors of the IEEE 802.16-2004 standard, the IEEE 802.16-2004/Cor1 (hereinafter, IEEE 802.16e) was completed in 2005 in a format of ‘corrigendum’.
Recently, in the IEEE 802.16 Task Group j (hereinafter, IEEE 802.16j) based on IEEE 802.16e, a relay station (RS) is introduced to provide coverage extension and throughput enhancement, and a standardization thereof is being carried out. According to the IEEE 802.16j specification, a signal can be delivered to a mobile station (MS) located outside a coverage of a base station (BS) via the RS, and a high-quality path employing high-level adaptive modulation and coding (AMC) can be configured for an MS located inside the coverage of the BS, thereby increasing system capacity with the same radio resource.
A representative operation mode of the RS includes a decode-and-forward (DF) mode and an amplify-and-forward (AF) mode. According to the DF mode, a wireless RS restores transmission information by performing various processes (e.g., demodulation, channel decoding, error checking, etc.) on a signal received from a source station, and thereafter delivers a signal generated by channel coding and modulation to a destination station. According to the AF mode, the wireless RS amplifies a signal received from the source station, and thereafter delivers the amplified signal to the destination station.
The DF mode has a merit in that noise can be removed, and reliability of signal transmission can be increased since errors can be checked. Further, a channel coding and modulation scheme can be modified by the RS. However, the DF mode also has a demerit in that a time delay may occur. On the other hand, the AF mode has a merit in that an operation of the RS is simple, and a time delay can be reduced. However, the AF mode also has a demerit in that noise included in a signal cannot be removed, and errors cannot be checked.
FIG. 1 shows a relay process according to an AF mode. FIG. 2 shows a relay process according to a DF mode. A transparent mode of IEEE 802.16j is used herein as an example.
Referring to FIG. 1 and FIG. 2, a frame n is temporally prior to a frame n+1. One frame includes a downlink (DL) region and an uplink (UL) region. The DL region includes a DL access zone and a DL transparent zone. The UL region includes a LL access zone and a UL relay zone. The DL access zone is a region where a base station (BS) transmits a signal to a relay station (RS) or a mobile station (MS). The DL transparent zone is a region where the RS transmits a signal to the MS. The UL access zone is a region where the MS transmits a signal to the RS or the BS. The UL relay zone is a region where the RS transmits a signal to the BS.
In FIG. 1, when the BS delivers a signal to the RS on the DL access zone of the frame n, the RS delivers a signal to the MS on the DL transparent zone of the frame n. Further, when the BS delivers a signal to the RS on the DL access zone of the frame n+1, the RS delivers a signal to the MS on the DL transparent zone of the frame n+1. As such, according to the AF mode, the RS receives a signal from the BS, and delivers the signal to the MS after only amplifying the signal. Thus, the signal can be delivered within the same frame without an additional time delay.
In FIG. 2, when the BS delivers a signal to the RS on the DL access zone of the frame n, the RS delivers a signal to the MS through the DL transparent zone of the frame n+1. As such, according to the DF mode, the RS performs modulation, decoding, and error checking on a signal received from the BS and thereafter performs coding and modulation, which leads to occurrence of an additional time delay. In addition, a size of signal may differ since a coding and modulation scheme changes.
To compensate for the aforementioned characteristics of the AF mode and the DF mode, there is an attempt to use the AF mode and the DF mode in combination with each other. For example, in a hybrid AF/DF scheme, a signal received by the RS is subjected to an information recovery process, and thereafter, if the recovery is successful, the signal is delivered according to the DF mode, and if the recovery is unsuccessful, the signal is delivered according to the AF mode. In this manner, a destination station can reduce a probability of reception errors by properly combining a signal received according to the AF mode with a signal retransmitted according to a hybrid automatic repeat request (HARQ) scheme or signals received through other paths.
The hybrid AF/DF scheme assumes that a modulation and coding scheme (MCS) level applied to a source station-relay (S-R) link is identical to an MCS level applied to a relay-destination station (R-D) link. However, in general, a channel condition of the S-R link is not identical to a channel condition of the R-D link. Therefore, different MCS levels may be applied to the S-R link and the R-D link, and allocation may be made by differentiating an amount of resources (e.g., bandwidth).
The hybrid AF/DF scheme is not effective when an applied MCS level and an amount of allocated resources are different in the two links. This is because the destination station prepares decoding according to an MCS level predefined for the R-D link, and thus cannot decode a signal received according to the AF mode. To deal with such a problem, the IEEE 802.16j standard specifies that a signal unsuccessfully received by the RS is discarded instead of being delivered to the destination station. However, this implies that resources pre-allocated to the R-D link are wasted when an error occurs in the S-R link.